1. Field of the Invention
The present invention relates to a control circuit for feeding an optimum decision threshold voltage to a clock and data recovery circuit, a decision threshold voltage controlling method, and a decision threshold voltage control program in the clock and data recovery circuit with a function of feeding a decision threshold voltage.
2. Description of the Related Art
In a recent optical transmission system including an optical amplifier or WDM (Wavelength Division Multiplexing) transmission technique, a factor that deteriorates an optical waveform is increased much more than in the conventional optical transmission system. The factor includes, by way of example, accumulation of noises caused by ASE (amplified spontaneous emission) by an optical amplifier, waveform distortion caused by dispersion and nonlinear effects in an optical fiber, which remarkably affects the quality of optical transmission with an increase of the optical power in the optical fiber, and crosstalk from the adjacent channels in the wavelength division multiplexing transmission.
In the optical waveform just after transmission, an eye opening, the area where an optical input signal can be surely distinguished into 1 or 0, is large, as shown in FIG. 19, while in the optical waveform after transmission of 600 km, the eye opening becomes very small, as shown in FIG. 25.
Even if such deterioration occurs in the optical waveform, a decision circuit of a receiving circuit is required to perform the optimum decision. Since the eye opening of the optical waveform varies depending on a light receiving power, the optimum decision threshold position for identifying 1 or 0 in the optical waveform varies depending on the light receiving power. The conventional optical transmission line, however, has some tolerance of the decision threshold position to the distortion of the optical waveform, and even if the decision threshold position is fixed at a value adjusted by a maker at a shipment of the product, it is no problem practically.
In the optical transmission system including an optical amplifier or WDM (wavelength division multiplexing) transmission, since a factor that deteriorates the optical waveform as mentioned above is increased, there occurs a curve in the error rate characteristic (floor) in the conventional decision circuit where the decision threshold position is fixed, which makes it difficult to assure the quality of a transmission line (upper portion of FIG. 26).
By consideration of the above situation, although various decision circuits for controlling a decision threshold position to the optimum one by way of hardware have been considered, it is necessary to design the parameters of a circuit minutely because the decision threshold position is processed through calculation, and the above circuits are defective in fine control.